Problem of Apollonius

The code solves the Apollonius problem using a simple algebric formula.

Apollonius' problem is to construct circles that are tangent to three given circles in a plane. Apollonius of Perga posed and solved this famous problem in his work Ἐπαφαί; this work has been lost, but a 4th-century report of his results by Pappus of Alexandria has survived.
Three given circles generically have eight different circles that are tangent to them and each solution circle encloses or excludes the three given circles in a different way.
The code takes the x,y coordinates of the centre of each circle and its radius as input. "cir=[x y r]"
If the radius of any circle is given a '-' sign, then that corresponding circle will lie inside the solution tangent circle, else it will be outside.